LIGNITEŚCOAL 309 this exists in several varieties. Sometimes it has the aspect and colour of wood (fossil wood) and sometimes it is brown, friable and easy to break (peaty, earthy lignite) ; in some cases it consists of superposed layers (schistose lignite) and in others is compact and varying in colour from brown to shining black (pitch-'coal). * * * When newly won, lignites contain 20-60 % of moisture, and when air-dried, 10-20%. The percentage of ash is very variable and, although it usually varies ;rom 2 to 15%, it may also be much greater. The elementary composition referred to the fuel free from ash and moisture, generally varies between the following limits : Carbon Hydrogen . Oxygen Nitrogen . 55-75% 4-7% 20-35% 0-5-2% In some lignites sulphur may be present in marked proportions. The calorific power of good lignites varies from 4,000 to 6,500 cals. Table XXXVII (see p. 310) gives the analytical results for various lignites. COAL Coal proper includes the bituminous coals and anthracite ; there is a gradual transition from the one to the other and no sharp delimitation. They constitute the most important industrial fuels. They are usually compact and black, the following types being distinguished: shining, black coal; opaque, black; cannel, of a velvety, blackish colour, with a conchoidal fracture ; fibrous coal; and bituminous slates (boghead). * * * According to their chemical composition, coals are classified, after Gruner, in six categories, which differ in the quantity and quality of the coke they furnish and in their calorific powers. The normal limits for each of these classes are indicated in Table XXXVIII (p. 311), the data in which are referred to the pure fuel (free from moisture and ash). more.